There are several types of display devices available for producing text or images for viewing. Presently, the most prevalent display types are Cathode Ray Tubes (CRTs), which make up the majority of desktop displays and televisions, and Liquid Crystal Displays (LCDs), which make up the majority of displays of portable devices, such as laptop computers, telephones, and personal digital assistants (PDAs).
Several other types of displays are not as well known, but are either available in limited quantities or are still being developed, such as plasma displays, Field Emission Displays (FEDs), Digital Light Processing (DLP) (a form of Micro Electro Mechanical Systems (MEMS)), Image Light Amplifiers (ILAs), and Light Emitting Diodes (LEDs). Each of these system types can generate displays for direct viewing, or can project an image on a surface for indirect viewing.
Many of these types of displays have problems with output uniformity from one picture element (pixel) to the others. For instance, in FED and LED systems, some individual pixels may generate more light for a given amount of driving signal than others. During the production of these displays, each pixel in the display is calibrated by individually lighting it and measuring the individual pixel's light intensity output. The measurements are compared to the output of the other pixels in the display. Adjustments are then made, such as by decreasing or increasing the drive signal when displaying that pixel, i.e., trimming (calibrating) the display circuitry for driving the pixels. This calibration can be done for a single level of the driving signal, or the driving signal can be varied throughout the entire gamma curve so that the pixels are measured at different levels of input, from being fully off to being fully on.
This uniformity test and calibration is typically done at manufacturing time, and the adjustments are usually permanently made. Therefore, if the output performance of the individual pixels changes over time, image quality of the display can degrade.
Another problem that some displays have is maintaining overall correct display brightness for the amount of light in the room. For instance, when the room itself is brightly lit, the display must be bright as well to be seen above the ambient light. When the room is dimly lit, a bright display would be overpowering and difficult to view, unless the display is darkened. Some displays can automatically adjust their brightness level in response to other light in the room. They do this by including a photosensor to sense the ambient room light, and then adjust the brightness of the entire display based on the sensor output. One problem with this system is that the sensor may be in an unusually bright or dim area compared to the majority of the display, giving a reading that is inaccurate for the entire display. For instance the sensor may be in a shadow, while the majority of the screen is in bright light. Using multiple sensors in various locations of the screen may help the problem, but this solution increases the complexity and the cost of the display. None of the sensors can be directly in front of the display otherwise they would cover the display. Therefore, no matter how many photosensors are included around the display, none of the sensors are actually measuring light on the display itself, but rather only measure light around the display screen.
Embodiments of the invention address these and other deficiencies in the prior art.